This invention relates to a light transmitting protective coating for electrical circuit assemblies.
Electrical circuits are conventionally mounted on a suitable circuit board or other support means having an appropriate electrical characteristic such as the necessary dielectric strength, power factor, insulating characteristic, and the like. The supporting structure must also have the necessary physical properties for a given application, such as tensile strength, tear resistance, and the like. In addition, in many instances the environmental useage must be considered in the design, construction and manufacture of electrical circuit systems. In particular, the humidity and/or temperature of the environment within which the electrical circuit must operate may have substantial significance in the design. In many environments, the circuit specification requires a protective enclosure such as a suitable coating, encapsulation, and the like. Both the electrical and physical characteristics of the enclosure must, of course, be considered in relationship to the environmental conditions to be encountered including rather extreme variations in the conditions in many instances. A coating which has the ability to transmit light from the exterior into the electrical circuit would be extremely useful. If repair or replacement is required, the coating can be removed and, after necessary maintenance, replaced. Thus, the coating permits visual inspection or review of the circuit as well as correction and revision of the circuit. If an opaque covering is used having the necessary electrical and other physical characteristics, the failure to transmit light may make it exceedingly difficult to maintain the covered circuit. One example in which the light transmission characteristic of the covering is significant is a badge reader for limiting of access to buildings or other controlled areas. Such device is necessarily generally mounted in the outside environment and subject to various changes in the environment including rain, snow, environmentally-borne salt and the like. The control circuit is advantageously sealed to protect the instrument against the environmental conditions. Generally such applications have required complete encapsulation within a suitable resin. A suitable protective resin which can be used as a sealant and/or as a conformal coating for environmental usage in varying temperature and humidity conditions, has not been found with both the desired light transmitting characteristic as well as all of the necessary electrical and physical characteristics required for effective and long life protection. Thus, the most carefully manufactured and trimmed systems generally have protrusions and asperities greater than 0.5 mil in height. Although materials such as urethane formulation has been suggested for many circuit applications because of its satisfactory electrical and physical properties, the material has not been found by the inventor to provide an effective coating of circuit boards with even such minute protrusions and asperities.
There is therefore no readily available coating material known to the present inventor which can be used as a circuit coating to effectively cover interconnecting projecting pin-like circuit elements which generally remain at least partially exposed with the use of a known coating. In such instances, the circuit is therefore embedded in a potting material to maintain the relatively minute protrusions and asperities completely encapsulated.